Purpose: To calibrate the inherent pixel variations of the on-board electronic portal imaging device (EPID) for the Halcyon (Varian Medical Systems, Palo Alto, CA) linear accelerator using a custom flattening phantom.
Methods: EPID images were acquired by irradiating a set of rectilinear solid water phantoms with various thicknesses (0-15 cm). Each solid water phantom was placed at the same position on the couch with a fixed source-to-couch distance (100 cm) and an open field set at the maximum size of 28 cm x 28 cm. Based on the EPID response as a function of solid water thickness, a quadratic form of the attenuation equation was fit with 2D parameter maps, Î±(x,y) and Î²(x,y), which represent the first- and second- order attenuation of a poly-energetic beam. With the 2D attenuation coefficient maps and the ratio of ideal EPID response of the 6FFF beam to a flattened beam, a 2D phantom thickness map was derived.
Results: On the central-axis, the beam required 66% attenuation for the flattening phantom. The final flattening phantom design had an overall conical shape with a maximum thickness of 5.6 cm, making it economical and lightweight. Regression values for the linear fits used in our approach ranged between 94% and 95% with a maximum absolute fit error of 0.0083.
Conclusion: Based on our results, it is possible to characterize pixel variations by designing a phantom that flattens the beam at the EPID. In the near future, this phantom will be milled to validate the flatness of the beam at the level of the EPID. Finally, the EPID response under this flattening phantom can be directly used as the new â€œflood fieldâ€? to correct the pixel response variations.
Funding Support, Disclosures, and Conflict of Interest: This research is supported by funding from Varian Medical Systems (VMS). Dr. Sasa Mutic receives consulting income from VMS for work outside of this project. This activity has been reviewed by Washington University's (WU's) Institutional Conflict of Interest Committee (ICOIC) in accordance with WU's Institutional Conflict of Interest (ICOI) Policy.